Multiple command objects for the same file

[abdelmaged]

I'm using clangd in vscode, and I'm working on a project where it's very common to compile the same file for different executables with different defines, i.e. if I have two executables exe1 & exe2, and one file file.c, then exe1 will compile file.c with -Dexe1 and exe2 will compile file.c with -Dexe2.

And file.c contents is something like:

int main() {
 // common code for exe1 & exe2 & ...

#ifdef exe1
  // something specific for exe1
#elif exe2
  // something specific for exe2
#else
  // something specific other executables
#endif

 // common code for exe1 & exe2 & ....

  return 0;
}

This produces compile_commands.json like:

[
  { "directory": "/home/user/llvm/build",
    "command": "gcc -c -o one/file.o -Dexe1 file.c",
    "file": "file.c" },

  { "directory": "/home/user/llvm/build",
    "command": "gcc -c -o two/file.o -Dexe2 file.c",
    "file": "file.c" },

  ...
]

We have one file file.c and multiple command object for the same file.

1. How does clangd handle this situation ?
2. How to tell clangd to dynamically pick specific command object for file.c, like pick the command with -Dexe2 instead of -Dexe1 ?
3. Is it possible that clangd can merge all command objects into single view ?

[HighCommander4]

1. How does clangd handle this situation ?

It will use an unspecified one of the available commands for the file (likely the first one it encounters).

2. How to tell clangd to dynamically pick specific command object for file.c, like pick the command with -Dexe2 instead of -Dexe1 ?

There isn't currently a way to do this.

One thing I have seen people do is perform a post-processing step on the compile_commands.json to filter out some entries. jq can be a helpful tool for this job.

3. Is it possible that clangd can merge all command objects into single view ?

It's not clear what merging would mean here.

I can imagine merging the commands, such that you get a command containing both -Dexe1 and -Dexe2. However, that can easily lead to compiler errors. For example:

#ifdef exe1
  int var = 42;
#elif exe2
  int var = 43;  // same variable name
#endif

If this is compiled with both -Dexe1 and -Dexe2, we would get a diagnostic like "mutiple declarations of variable var".

[i-ky]

I would like to add that regarding

Is it possible that clangd can merge all command objects into single view ?

...there is #681.